To choose the wattage of a basking bulb, one must consider the ambient temperature of the room and the species of cham. Some like it cooler than others.
One of the best things you can do, and I don't think anybody nobody touched on it above,
is to create multiple basking spots under a bulb.
Do this by having perches at different levels if at all possible.
This way,
if the Cham has had a big meal and needs to be extra warm to digest the big meal, he can sit on the perch that is say 9" under the bulb and is 88 degrees.
Or,
If he just wants to warm up at "sun-rise" to get moving and walk about the cage, then he could sit on the perch that is 13" under the bulb and say 84 degrees.
Dome lights are so inexpensive,
why not have two slightly different basking spots in a larger cage?
Do a small dome on one side with a 40 watt bulb in it, then have another dome in the opposite side with a 75w bulb in it.
That way the animal can choose what they want at any given time.

Just as they would in Nature.
I always vote for a TWO basking spot cage if the cage is large enough. To offer the animal more choices because they know what is best for them.
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Some other things to keep in mind:
1.
Spot light bulbs are good for making basking spots at far distances, but usually NOT the best to use in Cham cages.
2.
Flood light bulbs that point all the light down, but NOT into a tight focused spot are very good.
3.
A household bulb, or bulb of that traditional style is good in a dome fixture to create a larger basking area.
Cheers!
Todd
www.lightyourreptiles.com